Rewrite extra_source so that all supported sources are separate decorators

[kod-kristoff]

• Write decorators for all supported extra_source functions.
• Document when and how they are called
• For format extra sources, enable querying the backend of what formats are available.

Examples:

@ext.format({'<resource1>': ['html', 'tsv']})
@ext.format({'<resource2>': 'html'})
def function_named_freely(...):
  ...